1. FIELD OF THE INVENTION
This invention relates to a gain control circuit for controlling the gain of an amplifier by a control signal.
2. DESCRIPTION OF THE PRIOR ART
A prior art gain control circuit for controlling the gain of an amplifier by a control signal has generally had a construction as shown in FIG. 1. Referring to FIG. 1, such a prior art gain control circuit comprises a pair of transistors 1 and 2 constituting a differential amplifier, a constant-current transistor 3, resistors 4, 5 and 6, a control signal source 7, and a differential amplifier 8. In this prior art gain control circuit, the output of the control signal source 7 is made variable to vary the current flowing through the emitter resistor 6 for the transistor 3 thereby controlling the gain. The reference numerals 9 and 10 designate base resistors for the transistors 1 and 2 respectively.
While applying the control signal from the control signal source 7 to the base of transistor 3, an input signal Ei is applied to the base of transistor 1. Suppose that the gain of the differential amplifier 8 is unity, and its output is E.sub.o, then there is the following relation between E.sub.i and E.sub.o : EQU E.sub.o /E.sub.i = (q/kT) IR.sub.L1 ( 1)
where q is the electric charge of each electron, k is the Boltzmann's constant, T is the absolute temperature, I is the current flowing through the emitter resistor 6 for the transistor 3, and R.sub.L1 is the resistance value of the resistor 4. It will thus be seen that the gain can be controlled by the control signal since the gain varies in proportion to the current I flowing through the emitter resistor 6 for the transistor 3, and this current I is proportional to the output of the control signal source 7. The prior art having concern with such a gain control circuit is disclosed in, for example, "Electronic Design" No. 13, published June 21, 1974, pp. 94-99.
However, in such a gain control circuit, leakage of the control signal into the output E.sub.o of the differential amplifier 8 tending to extremely deteriorate the characteristic of the amplifier has generally been inevitable due to fluctuation of the resistance value of the resistors, and fluctuation of the base-emitter voltage and current amplification factor of the transistors.